Remove unused variables in faiss/IndexIVF.cpp

Summary:
LLVM-15 has a warning `-Wunused-but-set-variable` which we treat as an error because it's so often diagnostic of a code issue. Unused variables can compromise readability or, worse, performance.

This diff either (a) removes an unused variable and, possibly, it's associated code, or (b) qualifies the variable with `[[maybe_unused]]`, mostly in cases where the variable _is_ used, but, eg, in an `assert` statement that isn't present in production code.

 - If you approve of this diff, please use the "Accept & Ship" button :-)

Reviewed By: dmm-fb

Differential Revision: D56065763

fbshipit-source-id: b0541b8a759c4b6ca0e8753fc24b8c227047bd3d

demos/demo_imi_pq.cpp

@@ -77,7 +77,6 @@ int main() {
     // the coarse quantizer should not be dealloced before the index
     // 4 = nb of bytes per code (d must be a multiple of this)
     // 8 = nb of bits per sub-code (almost always 8)
-    faiss::MetricType metric = faiss::METRIC_L2; // can be METRIC_INNER_PRODUCT
     faiss::IndexIVFPQ index(
             &coarse_quantizer, d, ncentroids, bytes_per_code, 8);
     index.quantizer_trains_alone = true;

faiss/IndexIVF.cpp

@@ -444,7 +444,7 @@ void IndexIVF::search_preassigned(
         max_codes = unlimited_list_size;
     }
 
-    bool do_parallel = omp_get_max_threads() >= 2 &&
+    [[maybe_unused]] bool do_parallel = omp_get_max_threads() >= 2 &&
             (pmode == 0           ? false
                      : pmode == 3 ? n > 1
                      : pmode == 1 ? nprobe > 1
@@ -784,7 +784,7 @@ void IndexIVF::range_search_preassigned(
 
     int pmode = this->parallel_mode & ~PARALLEL_MODE_NO_HEAP_INIT;
     // don't start parallel section if single query
-    bool do_parallel = omp_get_max_threads() >= 2 &&
+    [[maybe_unused]] bool do_parallel = omp_get_max_threads() >= 2 &&
             (pmode == 3           ? false
                      : pmode == 0 ? nx > 1
                      : pmode == 1 ? nprobe > 1

faiss/utils/distances.cpp

@@ -141,7 +141,7 @@ void exhaustive_inner_product_seq(
         const IDSelector* sel = nullptr) {
     using SingleResultHandler =
             typename BlockResultHandler::SingleResultHandler;
-    int nt = std::min(int(nx), omp_get_max_threads());
+    [[maybe_unused]] int nt = std::min(int(nx), omp_get_max_threads());
 
     FAISS_ASSERT(use_sel == (sel != nullptr));
 
@@ -178,7 +178,7 @@ void exhaustive_L2sqr_seq(
         const IDSelector* sel = nullptr) {
     using SingleResultHandler =
             typename BlockResultHandler::SingleResultHandler;
-    int nt = std::min(int(nx), omp_get_max_threads());
+    [[maybe_unused]] int nt = std::min(int(nx), omp_get_max_threads());
 
     FAISS_ASSERT(use_sel == (sel != nullptr));